A significant number of potential pregnancies are lost during the periimplantation stages of embryo development. It has been estimated by the Commission on Life Sciences of the National Research Council that more than 200,000 infants born each year in this country have abnormalities that arise during embryonic development. Other infants have markedly low birth weights which may result in death or disability later in postnatal life. A large portion of these abnormalities may result from alterations of normal embryonic development. Our studies have indicated that nitric oxide (NO) is required for preimplantation embryo development. In addition, exposure to estrogen, which induced implantation, increases the production of NO 10 fold in dormant blastocysts. However, little is known about the affect of NO on gene expression in periimplantation embryos. The purpose of the proposed experiments is to elucidate the integrated mechanisms regulated by NO in early embryonic development and initiation of implantation. This will be accomplished by conducting the following experiments: (a) Compare the localization of expression of the nitric oxide synthase (NOS) genes in embryos on days 1-4 of pregnancy (preimplantation). Results will indicate if induction of more than one NOS gene is required for NO production and if there is differential expression of these genes during preimplantation embryo; (b) Determine the nature of signaling cascades generated by NO in the embryo on days 1-4 of pregnancy. Specifically we will examine the effect, time course and cell specificity of cell cycle genes including: early immediate genes: c-los, c-jun and e-myc (mRNA and protein), cyclins (D,E, A and B) and retinoblastoma gene in response to NO. Results will determine if NO is acting through the activation of immediate early genes to genes regulating the cell cycle: (c) Determine if the inhibition of NO production alters expression of cell-cycle checkpoint genes. Determine the effect of inhibition of NO on the time course and cell specificity of levels of the cell-cycle checkpoint genes ATM, ATR, MAD and BUB in embryos on days 1-4 of pregnancy. Results will indicate if inhibition of NO stops normal mitotic division by altering expression of checkpoint genes. The proposed study will establish the status of NO production in the embryo and its effects on embryonic gene expression will generate important and meaningful information regarding normal and abnormal reproductive functions. Furthermore, this information should have implications in human fertility treatments, as a tool to select healthier IVF-derived embryos with increased probability for successful pregnancy.